1. Field of the Invention
The present invention relates to a direct filling ink capable of being used in a ballpoint pen.
2. Description of Related Art
A ballpoint pen where ink is filled directly by an ink tube or a coloring agent is generally known. Typically, inks used in these types of pen have a shear-thinning property. In other words, they have a high viscosity when not being used, and a low viscosity in use due to rolling (shearing force) of the ballpoint pen. Such ballpoint pens tend to have many problems.
For example, one problem is that when the water content in the ink evaporates, the water soluble (dispersible) resin in the ink thickens and/or forms a layer on the pen point due to its considerable high molecular weight, thereby adversely affecting the quality of writing with the pen. The use of a cross-linked acrylic acid polymer ink having a relatively low thickening ratio and forming no layer when water content evaporates solves such problem However, such cross-linked acrylic acid polymer inks have low thixotropy. Thus, desired viscosity is not often obtained by shearing force during a writing operation. This causes bleeding and blurs the writing.
If a pigment is used as a coloring agent, the light resistance and water resistance in the writing tend to be excellent. However, when the water content in the ink evaporates, the pigment can not be dispersed, thereby clogging the ink on the pen point. Thus, it is often difficult to get the pen to write at the onset of each use, or the pen may be incapable of writing at all. And what is worse, in case of the pigment ink, once its dispersion is lost or the pigment has hardened, the pigment generally is difficult to redisperse even if new ink is added. Thus, it is typically difficult to restore. Thus, as of yet, there is no ink suitable for use in a ballpoint pen containing an aqueous ink composition that has sufficient writing ability and resistance to drying.
Due to such problems, ballpoint pens having aqueous ink compositions, must be equipped with a cap to prevent the evaporation of water from the ink. However, a capless type of ballpoint pen capable of commencing a writing operation at a single touch is urgently demanded.